We have found a significant positive correlation of cortisol levels with sexual performance in old rhesus males. We plan to determine blood levels of testosterone and cortisol, to carry out tests of sexual behavior in both old and young males at 0900 and 2100, and to determine correlations between cortisol levels and sexual performance in old and young males. The presence of a correlation between cortisol and sexual behavior does not imply a causative relationship. To further probe the relationship, we will increase corticosteroid levels by ACTH infections and determine whether sexual performance is altered in the males and whether young and old males respond differently. We will determine whether sexual behavior can be restored to previous levels in old males castrated several years earlier:; examine the effects of estradiol on sexual performance in these males; and compare feedback effects of estradiol and testosterone on luteinizing hormone (LH) levels. In studies on nonhormonal variables that influence the sexual performance of old rhesus males we will examine the effects of months without exposure to female on testosterone levels and on sexual behavior. In addition, we will explore further the significance of preferred females on the sexual performance of old male monkeys.